1. Field of the Invention
The present invention relates to a method for manufacturing a press-molded glass object such as a glass lens or a prism and apparatus therefor.
2. Description of the Related Art
Apparatus for manufacturing a press-molded glass object generally comprises a frame, a fixed shaft extending downward from the upper surface of the frame and fixed to the frame, a fixed mold attached to the lower end face of the fixed shaft, a mobile shaft opposing the fixed mold and vertically movably arranged, and a mobile mold attached to the upper end of the mobile shaft. In this apparatus, a glass material is arranged on the mobile mold, the mobile mold is moved upward to be in tight contact with the fixed mold, and the material is press-molded at a predetermined temperature, thereby manufacturing the object. Although a hydraulic cylinder or pneumatic cylinder is mainly used as a drive source for the mobile mold, noise or contamination generated by the drive source pose problems. In addition, when press molding is performed by only bringing the mobile mold and the fixed mold into tight contact with each other, shaping stability of the molded product, accuracy, and reproducibility are poor.
More specifically, a glass material is first heated. Subsequently, it is heated to a predetermined temperature, press-molded, and then cooled to a temperature at which the material is extracted from the apparatus. In this cooling process, the glass material decreases in size more than the molds do, due to the difference between the coefficients of thermal expansion. Due to this shrinkage, the optical surface cannot be formed with high precision. The coefficient of thermal expansion of glass is several times larger in the temperature region higher than the transition point than in the temperature region lower than the transition point.
In a molding apparatus wherein a servo motor is used as a drive source, a glass material is arranged between a pair of upper and lower molds, the molds and glass material are heated, and the material is press-molded, the following method is known. That is, when the glass material is pressed, the upper and lower molds are not moved to the final tight contact position; they are kept away from each other by the distance corresponding to the shrinkage of the glass material. Subsequently, the glass material is cooled until its temperature becomes close to the transition point. At such a temperature, the glass material is finally pressed to obtain an optical element. Such a method is described, for example, in Jpn. Pat. Appln. KOKAI Publication No. 4-260620 and U.S. Pat. No. 5,264,016.
For example, Japanese Unexamined Patent Publication No. 4-260620 discloses a method in which a glass material is arranged between a pair of upper and lower molds, the molds are located at a position S1 or S2 where the upper mold and the glass material have a gap or the upper mold is in slight contact with the glass material until the temperature of the molds reaches a transition point temperature T2 at which the glass material can be deformed or a set temperature T1 which is lower than a temperature T3 almost equal to the temperature T2 by a predetermined amount, the glass material is pressed and held by the pair of molds at a set pressure P1 when the temperature of the molds reaches the set temperature T1, it is detected on the basis of moving of the molds that the glass material begins to be deformed when the temperature of the glass material reaches the temperature T2 or T3, and the temperature of the molds is considered as the temperature T2 or T3.
U.S. Pat. No. 5,264,016 discloses a method for manufacturing glass lenses in which the mold clamping force is unable to deform the lens blank when the temperature of the lens blank is below a transition point of the lens blank and the mold clamping force is able to deform the lens blank when the temperature of the lens blank is above the transition point.
However, even if the servo motor is used as described above, positional control and torque control cannot be performed at once. This means the follows. That is, when a material having low viscosity is pressed by positional control, and the set torque is to be obtained, the positions of the molds inevitably move. In contrast to this, when the molds are controlled by torque, and the molds are moved to the set position, the torque inevitably varies. In addition, when torque control is performed, and the molds reach the set position, the positional control has priority over the torque control. For this reason, the next program is disadvantageously started.
More specifically, when the upper and lower molds are positionally controlled to a set position where the upper and lower molds do not reach the final tight contact position, the next program is started immediately after the molds reach the set position. Even if the molds are held at the set position by timer setting, when a press shaft to which the molds are connected contracts in the cooling step, the molds are held at the set position because of the positional control. However, since the positional control can cope with a change in temperature, the tight contact state between the glass material and the molds cannot be held. That is, a pressing force acting on the glass material varies, a gap having a width corresponding to an amount of contraction of the press shaft is formed between the upper and lower molds, and the glass material is not held between the upper and lower molds. For this reason, if the glass material in the gap moves, the glass material is pressed in an "offset" state when the final pressing process is performed. Therefore, a nondefective product cannot be obtained.
When a mobile mold is controlled by torque control, and the mold reaches the set position, the positional control has priority over the torque control. For this reason, the next program is disadvantageously started.
As a countermeasure against this, the following method is available. That is, when the set position is set at a position (virtual tight contact position) above the actual mold tight contact position to prevent the mold from reaching the actual set position, the torque control can cope with a change in temperature. However, since the torque value exceeds the set torque value, the pressing force cannot be controlled. When the viscosity of the glass material is low, the mold may move to the final tight position because of the absence of resistance, and the thickness of a glass molded product cannot be controlled.
As described above, in the method of pressing the glass material without causing the upper and lower molds to reach the final tight position and then finally molding the material after cooling, it is impossible that the molds are kept being separated from the molds without changing the thickness of the glass in the molding process. That is, in the method above, a molded glass object (optical element) having high accuracy and no reproducibility in the thickness of the molded product cannot be obtained.
In addition, since a frame 1, a fixed shaft 2, and a mobile shaft 9 thermally expand in a heat-molding process, the following problem is posed in the thickness of a molded product. FIG. 10A shows a state wherein the temperature of the overall apparatus is T, FIG. 10B shows a state wherein the temperature of the overall apparatus is T+dT. When the temperature of the overall apparatus is T+dT, assume that the length of the frame 1 is L.sub.F +dL.sub.F ; the length of the fixed shaft 2, L.sub.AU +dL.sub.AU ; the length of the mobile shaft 9, L.sub.AL +dL.sub.AL ; and the thickness of the glass element, t+dt. In this case, EQU L.sub.F +dL.sub.F +(L.sub.AU +dL.sub.AU)+(L.sub.AL +dL.sub.AL)+(t+dt)
is satisfied. In this case, due to the differences among the thermal expansion coefficients of the frame 1, the fixed shaft 2, and the mobile shaft 9, the following relationship is established: EQU .vertline.dL.sub.F .vertline..noteq..vertline.dL.sub.AU +dL.sub.AL .vertline.
For this reason, EQU dt=.vertline.dL.sub.F .vertline.-.vertline.dL.sub.AU +dL.sub.AL .vertline.t.noteq.t+dt
is satisfied.
A limit switch 29 attached to an intermediate plate 1a gives an origin position Z.sub.0 to the mobile shaft 9, a moving distance Z.sub.1 from the origin position Z.sub.0 is accurately controlled by positional control performed by a numerical control mechanism (NC). For this reason, although the thickness of the glass element is set to be t, the thickness of the glass element at a temperature T+dT is t+dt. The value dt indicates a slight amount of about several tens .mu.m, but is important value for discriminating standardized products from non-standardized products when higher accurate glass elements are manufactured.